Tachycardia in Pregnancy

A 32-year-old woman who is 14 weeks pregnant (gravida 1, para 0) presents with a 6-week history of exertional palpitations and fatigue. She reports severe palpitations that worsen with activity and are associated with lightheadedness and fatigue. She experiences significant palpitations on standing after lying down for a while. The palpitations improve with lying flat. Her symptoms have significantly limited her level of activity. Her pregnancy has otherwise been uneventful except for nausea and vomiting in the first few weeks. She has hypermobile Ehlers-Danlos syndrome (hEDS) and migraine headaches. She reports no smoking history or alcohol use. She drinks approximately 1.78 L (60 oz) of nonalcoholic fluid daily.

Her vital signs include resting heart rate (HR) 92 bpm taken in a supine position and 136 bpm after 8 min of standing. Respiratory rate is 16 breaths/min, and blood pressure (BP) is 112/72 mm Hg in a supine position and 100/68 mm Hg after standing for 8 min. Physical examination reveals no abnormalities. S1 and S2 are normal; no other heart sounds are heard. There are no murmurs, rubs, or gallops.

Laboratory study results, including complete blood count (CBC) values, electrolyte panel values, and thyroid-stimulating hormone level, are within the reference range. Electrocardiogram (ECG) findings are unremarkable.

Which one of the following is the most likely diagnosis?

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